In addition to Ed's suggestions, a couple of things off the top of my head:
- When you are playing, are you putting a lot of pressure on the thumb that's anchored to the thumbrest? If so, that could be the problem.
- What kind of angle is your wrist at while playing? The straighter your wrist is, the better. If it's sitting long periods of time at a severe angle, that could be the problem.
So, give it a rest for a few days, and I like Ed's suggestion of warm water and a massage. Then, start observing your hand while playing; it should be loose and comfortable. Observe how much pressure you are putting on your thumb, and observe at how much of an angle the wrist is bent. Experiment with changes.